Why did Virgin Mobile take no action about sim-card fraud?

The network is billing us for cards we do not have – to a total of more than £500 so farMy mother-in-law has been charged by Virgin Mobile for sim cards she doesn’t possess. It has been going on since September 2016 – a total of £546. Over two weeks ago I raised this with the company and was advised I would be contacted within five working days. Her bank refunded the full amount under the direct debit guarantee. I never received a call back from Virgin’s fraud